Delayed cokers are furnace-type coking units wherein the feed is rapidly heated to temperatures above coking temperature inside a furnace and the effluent from the furnace discharges (before decomposition) into a large “coke drum”, where it remains until it either cracks or thermally decomposes and passes off as vapor and also condenses into coke.
In the usual application of the delayed coking process, residual oil is heated by exchanging heat with liquid products from the coking process and is then fed into a fractionating tower where any light products which might remain in the residual oil are distilled out and also mixes with the internal recycle fraction. The oil is then pumped through a furnace where it is heated to the required temperature and discharged into the bottom of the coke drum. The first stages of thermal decomposition reduce this oil to a very heavy tar or pitch which further decomposes into solid coke. The vapors formed during this decomposition produce pores and channels in the coking mass through which the incoming oil from the furnace may pass. This process continues until the drum is filled with a mass of coke. The vapors formed in the process leave from the top of the drum and are returned to the fractionating tower where they are fractionated into desired cuts.
The delayed coking heater outlet temperature is controlled in the temperature range of 900° to 950° F. Higher temperatures may cause rapid coking in the coking heater and shortened on-stream time. Lower temperatures produce soft coke with a high VCM content. Sufficient pressure to avoid vaporization of the feed is maintained in the heater. The residence time must be long enough to bring the oil up to the desired temperature but excess time in the heater may cause coking and result in clogging the heater coil. A method frequently used for controlling the velocity and residence time in the heating coil is to inject water (or steam) into the high-boiling petroleum oil entering the heating coil. Water or steam injection is controlled at a rate sufficient to maintain the oil velocity in the heating coil to prevent coke from forming and depositing in the coil.
Coke formation reactions are essentially endothermic with the temperature dropping to 780° to 900° F., more usually to 780° to 840° F., in the coke drum. Coke drum pressures are maintained in the range from 10 to 70 psig. To avoid the temperature limitations of delayed coking units, both moving bed and fluidized bed units have been proposed for reduced crude coking operations. Because they generally operate at lower pressures and higher temperatures than delayed cokers, more of the feed charge to fluid and contact or moving bed cokers is vaporized. The higher temperatures of fluid and contact or moving bed units also result in higher octane gasoline than that from delayed coking and in more olefinic gases. However, despite the development of these higher temperature coking processes, most commercial coking operations currently employ the delayed coking process.
The principal charging stocks for coking operations are high boiling virgin or cracked petroleum residues which may or may not be suitable as heavy fuel oils. Most of the delayed cokers in operation around the world produce fuel grade coke, which is used as an industrial fuel. Fuel grade coke prices are much lower compared to other products from coker units. Some delayed cokers produce anode grade coke for making electrodes used in aluminium industries. Prices of anode grade coke are higher compared to fuel grade coke but still lesser compared to other products from coker. Therefore, it is highly desirable to have a process which can effectively reduce the generation of coke from delayed coking process to improve the margin around the delayed coker.
Various additives have been tried in the past for reducing the yield of coke and improving the lighter product yields in delayed coking process. For example, U.S. Pat. No. 4,378,288 discloses the use of free radical inhibitors like benzaldehyde, nitrobenzene, aldol, sodium nitrate etc. with a dosage of 0.005-10.0 wt % of the feedstock which majorly has been vacuum tower bottom, reduced crude, thermal tar or a blend thereof.
Similarly, U.S. patent publication No. 2009/0209799 discloses FCC catalysts, zeolites, alumina, silica, activated carbon, crushed coke, calcium compounds, Iron compounds, FCC Ecat, FCC spent cat, seeding agents, hydrocracker catalysts with a dosage of <15 wt % of the feed which is majorly a suitable hydrocarbon feedstock used in delayed coking of boiling point higher than 565° C. to obtain a reduction in coke yield of about 5 wt %.
U.S. Pat. No. 7,425,259 discloses a method for improving the liquid yields during thermal cracking using additives. Additives such as metal overbases of Ca, Mg, Strontium, Al, Zn, Si, Barium were used.
From the prior arts, it can be seen that an additive or a combination of additives or catalysts are being used to alter the reaction mechanism and achieve the yield improvement. It is notable that many of the additives and catalysts involve additional cost of usage. Also, their impacts on the quality of coke as well as other products are not discussed in detail in the prior arts. It is also possible that the metallic additives get trapped in the solid carbonaceous coke, increase the ash content rendering the product un-usable. Therefore, it is desirable to have a process capable to improve the yield pattern from the thermal cracking process, without the use of any forms of external additives.